This invention relates to a paste which contains an electrically conducting powder which is useful to form a conducting solid filler in a cavity in a ceramic substrate of an electric or electronic device having at least two conducting parts separately fixed to the substrate such that the solid filler provides electrical connection between these conducting parts, and a method of filling such a cavity with a conducting solid filler by using a paste according to the invention.
There are many electric or electronic devices having a substrate of a ceramic material, and in some of such devices a hole or cavity is formed in the ceramic substrate and filled with an electrically conducting solid filler such that the filler serves as a junction between two, or more, conducting parts separately fixed to the substrate.
For example, there is a device having thin electrode layers deposited or printed on a major surface of a ceramic substrate and two thin lead wires which are partly embedded in the substrate to enhance mechanical strength. In such a device, electrical connection between each electrode layer and one of the lead wires is usually established by the following technique. Initially, a cavity or hole is formed in the substrate in a region covered with one of the electrode layers and another hole in a separate region covered with the other electrode layer. The end portions of the two lead wires are disposed in these two holes, respectively, and subsequently these two holes are filled up with an electrically conducting solid filler which remains in close contact with both the lead wire and the electrode layer. With the view to achieving complete filling of the respective holes, which are usually very small in dimensions, without damaging the lead wires and with good contact of the resultant solid filler with the electrode layer and lead wire in each hole, it is a usual method to form the conducting solid filler by first filling up each hole with a paste containing a metal powder and subsequently firing the substrate to cause sintering of the metal powder in the hole into a solid mass.
In practice, a typical example of pastes for this method is a dispersion of a mixture of a platinum powder and a glass powder in a liquid vehicle which is a solution of an organic polymer in a solvent. However, the result is often unsatisfactory because the paste in the hole undergoes significant shrinkage during the sintering process so that there are considerable gaps between the solid filler and the lead wire and wall surfaces defining the hole, whereby the electrical connection between the lead wire and the electrode layer becomes too high in resistance and low in reliability. Sometimes a microcrystalline wax is added to the above described paste composition with the intention of lessening the degree of shrinkage, but the effect of the addition of such a wax is limited and insufficient to ensure satisfactory results. Besides, when the ceramic substrate is initially prepared as a so-called green sheet and sintered afrer filling of the aforementioned holes with the paste, the wax contained in the paste is liable to exert a detrimental influence on the physical properties of the substrate. In addition, the high temperatures needed to melt the wax at the stage of filling the holes with the paste might be detrimental to the binder contained in the substrate as a green sheet.